The increasing wealth of nations is accompanied by a fall in fertility, so that in many developed (and developing) nations, fertility rates have dropped below the replacement value of about 2.1 births per woman. This ‘birth dearth’, together with the aging of populations, presents many difficult social and political problems. But, based on new cross-sectional and longitudinal analyses of the relationship between the total fertility rate and the human development index, Myrskylä et al. show that above a certain degree of economic development, fertility once again begins to rise, slowing the rate at which populations age. As a consequence, in contrast to the current popular and scientific debates, it seems likely that countries at the most advanced development stages will face a relatively stable population size, if not an increase in total population in cases where immigration is substantial.

I chatted with one of the authors, Hans-Peter Kohler of the University of Pennsylvania, and he explained that this finding doesn’t appear to influence the global trajectory toward a population peak of more or less 9 billion because the level of development where the uptick in fertility occurs is very high. The global population peak will still largely be determined by changes in birth and death rates in developing countries that, for the moment, remain deeply impoverished. But the finding does imply that anyone who expects a flattening or shrinking of populations in the world’s richest countries will blunt their impact on the environment through greenhouse-gas emissions or other factors is bound to be disappointed, he said. He also mentioned that immigration, particularly in the United States, is another major driver of population trends (as Joseph Chamie noted here).

[UPDATE 9/09: Mark Lauer, an independent risk analyst, has done a fascinating critique of the data crunching in the paper, including animated graphs showing a marked lack of an uptick in fertility. I’ve forwarded the post to Dr. Kohler and will do a fresh post on all of this soon.]

Here’s Dr. Kohler’s comment on the environmental implications of rising fertility in the world’s most developed countries:

World population growth is primarily driven by developing countries, and the only highly developed country that significantly contributes to global population growth is the United States. (eg., Chap 1 in Population Reference Bureau (2004) Transitions in World Population Population Bulletin, 59(1), 3-40, available at http://www.prb.org/Source/ACFFF4.pdf). The reversal of the fertility rates with development, which we document in our Nature paper, occurs among the most-developed countries that were — with the exception of the United States — projected to shrink, stabilize or grow only very moderately in terms of their population size in the next decades. The future of the size of the world population will continue to be driven primarily be the pace of future fertility decline (and by mortality conditions) among developing countries. For these countries, our study confirms the existing notion that continued development will likely help to reduce fertility. So, we expect based on our finding that continued development will reduce fertility rates in all but the most advanced countries, and that development will thus slow the rates of global population growth.

The reversal of fertility declines with further H.D.I. increases among the most developed countries will have no big impact of this overall prospect for the world population size. The future of the world population size will mostly be determined by the developing countries, and not by the countries for which we document a reversal in fertility trends.

Of course, even a modest increase in fertility rates among the most developed countries will imply that the population size in these countries declines less rapidly, or grows somewhat more rapidly than would otherwise have been the case. One could argue — on some people do — that, from the perspective of global resource consumption, a world with fewer individuals living in highly developed countries — ie, the places where per capital resource consumption is highest — would be desirable in order reduce the climate impact and resource consumption of the human population. But it turns out that the transition from a growing population to a relatively rapidly declining population is problematic. A decline in population size implies rapid population aging and a size of the labor force that declines — in the short term — more rapidly than the overall population size. Many countries have found these prospects to be very challenging, and many have argued that the rates of population aging and population decline that is projected for many European and other very low fertility countries is not sustainable: it implies rather insurmountable problems for social security and other transfer systems, and it may lead to declines in well-being as a result of lower productivity and incomes. The findings in our paper help to ameliorate some of these concerns, and they point to the possibility that — among the most developed countries — further progress may actually result in somewhat higher fertility. But even with this reversal, in most advanced societies fertility will remain below (and sometime, substantially below) replacement level (about 2.1 children per woman). For example, even with the reversal in the T.F.R. [total fertility rate] during the last decade in Italy, the Italian T.F.R. is just slightly above 1.3 children per women; in the US, it is slightly above 2 children per woman.

The challenges that the world faces in the next decades regarding climate change will likely have to be addressed by changing consumption patterns and technological innovations; changes in population size in the most developed countries, with the possible exception of the United States, is a relatively slow process that occurs over many decades and that will unlikely to have a significant impact on how the world resolves the challenges of climate change and resource constraints.

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By 2050 or so, the human population is expected to pass nine billion. Those billions will be seeking food, water and other resources on a planet where humans are already shaping climate and the web of life. Dot Earth was created by Andrew Revkin in October 2007 -- in part with support from a John Simon Guggenheim Fellowship -- to explore ways to balance human needs and the planet's limits.